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          <h1 class="post-title" itemprop="name headline">函数式编程基础(1)</h1>
        

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<h3 id="定义"><a href="#定义" class="headerlink" title="定义"></a>定义</h3><p>函数式编程不是用函数来编程, 也不是传统的面向过程编程, 其主旨在于将复杂的函数复合成简单的函数.</p>
<p>以下例子是用函数写代码, 并非是函数式编程</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br></pre></td><td class="code"><pre><span class="line">function test()&#123;&#125;</span><br><span class="line">test();</span><br></pre></td></tr></table></figure>

<h3 id="特性"><a href="#特性" class="headerlink" title="特性"></a>特性</h3><ol>
<li>函数是”第一等公民”</li>
<li>只用”表达式”, 不用”语句”</li>
<li>没有”副作用”</li>
<li>不修改状态</li>
<li>引用透明(函数运行只靠参数)</li>
</ol>
<h3 id="优缺点"><a href="#优缺点" class="headerlink" title="优缺点"></a>优缺点</h3><p>函数式编程的一个明显的好处是这种声明式的代码, 对于无副作用的纯函数, 我们完全可以不考虑函数内部是如何实现的, 专注于编写业务代码. 优化代码时, 只需要集中在这些稳定坚固的函数内部即可.<br>与此相反, 不纯的函数式的代码会产生副作用, 或者依赖外部系统环境, 使用的时候总要考虑这些不干净的副作用.</p>
<h3 id="术语"><a href="#术语" class="headerlink" title="术语"></a>术语</h3><ol>
<li><p><strong>纯函数</strong>: 对于相同的输入, 永远会得到相同的输出, 而且没有任何可观察的副作用, 也不依赖外部环境的状态</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br></pre></td><td class="code"><pre><span class="line">var test = [1,2,3,4,5];</span><br><span class="line">// Array.slice是纯函数, 因为它没有副作用, 对于固定的输入, 输出总是固定的.</span><br><span class="line">test.slice(0 ,3);</span><br><span class="line">test.slice(0 ,3);</span><br><span class="line">test.splice(0 ,3); // 会破坏原来数组的值</span><br><span class="line">test.splice(0 ,3);</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">// lodash的memoize函数</span><br><span class="line">import _ from &apos;lodash&apos;;</span><br><span class="line">var sin = _.memoize(x =&gt; Math.sin(x));</span><br><span class="line">var a = sin(30); // 第一次执行的时候会稍微慢点</span><br><span class="line">var b = sin(30); // 第二次有了缓存, 速度极快</span><br><span class="line">// 纯函数不仅可以有效降低系统的复杂度, 还有很多很棒的属性, 比如可缓存性</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">// checkAge的纯与不纯</span><br><span class="line">var min = 20;</span><br><span class="line">var checkAge = age =&gt; age &gt; min;</span><br><span class="line">var checkAge = age =&gt; age &gt; 20; // 纯的, 这很函数式</span><br><span class="line">// 在不纯的版本中, checkAge不仅取决于age, 还有外部依赖的变量min</span><br><span class="line">// 纯的checkAge把关键字&apos;20&apos;硬编码在函数内部, 扩展性比较差, 柯里化优雅的函数式解决.</span><br></pre></td></tr></table></figure>
</li>
<li><p><strong>函数的柯里化</strong>: 传递给函数一部分参数来调用它, 让它返回一个函数去处理剩下的参数</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br></pre></td><td class="code"><pre><span class="line">var checkAge = min =&gt; (age =&gt; age &gt; min);</span><br><span class="line">var checkAge30 = checkAge(30); // 闭包缓存变量</span><br><span class="line">checkAge30(40);</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">// 柯里化之前</span><br><span class="line">function add(x, y) &#123; return x + y; &#125;</span><br><span class="line">add(1, 2); // 3</span><br><span class="line"></span><br><span class="line">// 柯里化之后</span><br><span class="line">function addX(y) &#123;</span><br><span class="line">  return function(x) &#123;</span><br><span class="line">    return x + y;</span><br><span class="line">  &#125;</span><br><span class="line">&#125;</span><br><span class="line">addX(2)(1); // 3</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">// 函数的柯里化</span><br><span class="line">function foo(p1, p2) &#123;</span><br><span class="line">  this.val = p1 + p2;</span><br><span class="line">&#125;</span><br><span class="line">var bar = foo.bind(null, &apos;p1&apos;);</span><br><span class="line">var baz = new bar(&apos;p2&apos;);</span><br><span class="line">console.log(baz.val); // p1p2</span><br><span class="line"></span><br><span class="line"></span><br><span class="line">import &#123; curry &#125; from &apos;lodash&apos;;</span><br><span class="line">var match = curry((reg, str) =&gt; str.match(reg));</span><br><span class="line">var filter = curry((f, arr) =&gt; arr.filter(f));</span><br><span class="line">var haveSpace = match(/\s+/g);</span><br><span class="line">// haveSpace(&apos;fffff&apos;);</span><br><span class="line">// haveSpace(&apos;a b&apos;);</span><br><span class="line"></span><br><span class="line">// filter(haveSpace, [&apos;abcd&apos;, &apos;hello world&apos;]);</span><br><span class="line">filter(haveSpace)([&apos;abc d&apos;, &apos;hello world&apos;]);</span><br><span class="line">事实上柯里化是一种&quot;预加载&quot;函数的方法, 通过传递较少的参数, 得到一个已经记住了这些参数的新函数.</span><br><span class="line">从某种意义上来讲, 这是一种对参数的&quot;缓存&quot;, 是一种高效的编写函数的方法.</span><br></pre></td></tr></table></figure>
</li>
<li><p><strong>函数组合</strong>: 纯函数以及如何把它柯里化写出的洋葱代码h(g(f(x))), 为了解决函数嵌套问题, 我们需要用到”函数组合”</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">var compose = (f, g) =&gt; (x =&gt; f(g(x)));</span><br><span class="line">var first = arr =&gt; arr[0];</span><br><span class="line">var reverse = arr =&gt; arr.reverse();</span><br><span class="line">var last = compose(first, reverse);</span><br><span class="line">last([1,2,3,4,5]); // 5</span><br></pre></td></tr></table></figure>

</li>
</ol>
<img src="https://ae01.alicdn.com/kf/HTB1vjCWdBKw3KVjSZFO761rDVXac.png" width="60%" height="60%" alt="compose-function" title="compose-function" style="margin-left: 40px">

<ol start="4">
<li><p><strong>Point Free</strong>: 把一些对象自带的方法转化成纯函数, 不要命名转瞬即逝的中间变量</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br></pre></td><td class="code"><pre><span class="line">const f = str =&gt; str.toUpperCase().split(&apos;&apos;);</span><br><span class="line">// 这个函数中, 我们使用了str作为中间变量, 但是这个中间变量除了让代码变的长了以外是毫无意义的</span><br><span class="line"></span><br><span class="line">const compose = (f, g) =&gt; (x =&gt; f(g(x)));</span><br><span class="line">const toUpperCase = value =&gt; value.toUpperCase();</span><br><span class="line">const split = x =&gt; (str =&gt; str.split(x));</span><br><span class="line">const fn = compose(split(&apos; &apos;), toUpperCase);</span><br><span class="line">fn(&apos;ab cde&apos;);</span><br></pre></td></tr></table></figure>
</li>
<li><p><strong>声明式与命令式代码</strong>: 命令式代码指我们通过编写一条又一条指令让计算机执行一些动作, 这其中可能会涉及到一些很复杂的细节. 而声明式代码相对优雅, 它是通过表达式的方式来声明, 执行的操作, 而不是通过一步步的指示</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line">// 命令式</span><br><span class="line">let testArrs = [];</span><br><span class="line">const companies = [&#123;name: 1&#125;, &#123;name: 2&#125;, &#123;name: 3&#125;, &#123;name: 4&#125;];</span><br><span class="line">for (var i = 0; i &lt; companies.length; i ++) &#123;</span><br><span class="line">  testArrs.push(companies[i].name);</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line">// 声明式</span><br><span class="line">let testArrs = companies.map(item =&gt; item.name);</span><br></pre></td></tr></table></figure>
</li>
<li><p><strong>惰性函数</strong>: 惰性函数(载入)就是if分支只会执行一次, 之后调用函数时, 直接进入所支持的分支代码</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br></pre></td><td class="code"><pre><span class="line">// DOM 事件添加中, 为了兼容现代浏览器和 IE 浏览器, 我们需要对浏览器环境进行一次判断</span><br><span class="line">function addEvent (type, el, fn) &#123;</span><br><span class="line">    if (window.addEventListener) &#123;</span><br><span class="line">        el.addEventListener(type, fn, false);</span><br><span class="line">    &#125;</span><br><span class="line">    else if(window.attachEvent)&#123;</span><br><span class="line">        el.attachEvent(&apos;on&apos; + type, fn);</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line">// 问题在于我们每当使用一次 addEvent 时都会进行一次判断, 可以利用惰性函数</span><br><span class="line">function addEvent (type, el, fn) &#123;</span><br><span class="line">    if (window.addEventListener) &#123;</span><br><span class="line">        addEvent = function (type, el, fn) &#123;</span><br><span class="line">            el.addEventListener(type, fn, false);</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br><span class="line">    else if(window.attachEvent)&#123;</span><br><span class="line">        addEvent = function (type, el, fn) &#123;</span><br><span class="line">            el.attachEvent(&apos;on&apos; + type, fn);</span><br><span class="line">        &#125;</span><br><span class="line">    &#125;</span><br><span class="line">&#125;</span><br></pre></td></tr></table></figure>

</li>
</ol>
<h3 id="高阶函数"><a href="#高阶函数" class="headerlink" title="高阶函数"></a>高阶函数</h3><p>函数当参数, 把传入的函数做一个封装, 然后返回这个封装函数, 达到更高程度的抽象.</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br></pre></td><td class="code"><pre><span class="line">// 命令式</span><br><span class="line">var add = function(a, b) &#123;</span><br><span class="line">  return a + b;</span><br><span class="line">&#125;;</span><br><span class="line"></span><br><span class="line">function math(func, array) &#123;</span><br><span class="line">  return func(array[0], array[1]);</span><br><span class="line">&#125;</span><br><span class="line"></span><br><span class="line">math(add, [1, 2]);</span><br></pre></td></tr></table></figure>

<h3 id="闭包"><a href="#闭包" class="headerlink" title="闭包"></a>闭包</h3><p>缓存了当前执行环境的词法作用域(定义函数时的作用域).</p>
<figure class="highlight plain"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br></pre></td><td class="code"><pre><span class="line">function fn1(test1) &#123;</span><br><span class="line">  // 词法作用域</span><br><span class="line">  function fn2(test2) &#123;</span><br><span class="line">    return test1 + test2;</span><br><span class="line">  &#125;</span><br><span class="line">  return fn2;</span><br><span class="line">&#125;</span><br><span class="line">var test = fn1(1);</span><br><span class="line">test(2);</span><br></pre></td></tr></table></figure>


      
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